Aggregation of the receptors with high affinity from immunoglobulin E (IgE) on mast cells and related cells by antigen leads to release of granules (exocytosis) and their contents. In addition, the aggregated receptors are internalized and they, the IgE bound to them and the antigen are degraded. In this project an attempt is being made to understand the mechanisms by which the receptors are incorporated into the plasma membrane and after stimulation, internalized. Advantage is taken of the fact that some of the same receptors that are not specifically aggregated can be followed simultaneously as can receptors for other Ig such as IgG. The principal findings during the current period has been evidence that there are cellular mechanisms by which the different receptors and receptor configurations can be discriminated.